1. Field of the Invention
The present invention concerns the field of prevention of explosions of electric transformers cooled by a volume of combustible fluid.
2. Description of the Relevant Art
Electric transformers sustain losses both in the windings and in the iron part, requiring the heat produced to be dissipated. Thus, high-power transformers are generally cooled by a fluid such as oil. The oils used are dielectrics and are capable of igniting above a temperature of about 140° C. Since transformers are very expensive devices, particular attention must be paid to protecting them.
An insulation fault generates, in the first instance, a strong electric arc prompting action by the electrical protection systems which trigger the transformer power cubicle (circuit breaker). The electric arc also results in a diffusion of energy causing gases to be discharged through decomposition of the dielectric oil, in particular hydrogen and acetylene.
After the gas discharge, the pressure inside the transformer tank increases very rapidly, leading to an often very violent deflagration. The deflagration causes significant tearing of the mechanical linkages of the transformer tank (bolts, welds) placing the said gases in contact with the oxygen in ambient air. Since acetylene self-ignites in the presence of oxygen, a fire breaks out immediately and spreads to other items of equipment on the site which are also likely to contain large quantities of combustible substances.
Explosions are caused by insulation ruptures due to short-circuits caused by overloads, voltage surges, a gradual deterioration of the insulation, insufficient oil level, the appearance of water or mould, or a failure of an insulation component.
There are known, in the prior art, fire extinguishing systems for electric transformers which are activated by fire detectors. However these systems operate with a significant lag, when the transformer oil is already burning. It was therefore accepted to merely restrict the fire outbreak to the equipment concerned so as not to spread the fire to the neighbouring installations.
To slow down the decomposition of the dielectric fluid due to an electric arc, silicon oils can be used instead of conventional mineral oils. However, the explosion of a transformer tank due to the increase in internal pressure is delayed for only an extremely short time period, in the order of a few milliseconds. This time period did not allow prevention of the explosion. Silicon oils are expensive.
There is known through document WO-A-97/12379 a method for preventing explosion and fire in an electric transformer equipped with a tank filled with combustible coolant fluid, by the detection of a rupture in the electrical insulation of the transformer using a pressure sensor, depressurization of the coolant fluid contained in the tank using a valve, and cooling of the hot parts of the coolant fluid by injecting a pressurized inert gas in the bottom of the tank in order to agitate the said fluid and to prevent oxygen from penetrating the transformer tank. This method is satisfactory and prevents the transformer tank from exploding.
Document WO-A-00/57438 discloses a rupture element with rapid opening for an electric transformer explosion prevention device.